(1) Field of the Invention
This invention relates to a diversity antenna system with electrical tilt, i.e. a phased array antenna system with space diversity and electrical tilt capability. It is relevant to many antenna systems such as those used for telecommunications, for example cellular mobile radio networks commonly referred to as mobile telephone networks. Such networks include second generation (2G) mobile telephone networks implemented by a cellular radio standard such as GSM and third generation (3G) mobile telephone networks implemented by a cellular standard such as the Universal Mobile Telephone System (UMTS). Other cellular radio standards include IS95 and WCMA2000.
(2) Description of the Art
Operators of conventional cellular radio networks generally employ their own base stations each of which is connected to one or more antennas. In such a network, the antennas are a primary factor in defining the desired coverage area which is generally divided into a number of overlapping cells, each associated with a respective antenna and base station. Each cell contains a fixed-location base station which maintains radio communication with mobile radios in that cell. Base stations are interconnected with one another for communication purposes, e.g. by fixed land lines arranged in a grid or meshed structure, allowing mobile radios throughout the cell coverage area to communicate with each other as well as with the public telephone network outside the cellular mobile radio network.
To improve and optimise communications performance in antenna systems used in cellular radio networks, it is known to use three techniques, space diversity, polarisation diversity and variable electrical tilt. The first such technique, i.e. space diversity, involves using two spaced apart antennas each giving rise to a respective received signal from a remote subscriber using a mobile telephone handset: this makes two received signals available from a single handset thereby providing a communications advantage.
Signals received by a base station undergo rapid fluctuations in power because they have traversed multipath environments: i.e. replicas of a signal transmitted from a mobile telephone handset pass via multiple paths or routes to a receiving base station, and the handset itself is in motion during transmission. Moreover, such replicas become decorrelated with one another. Signals received by a base station may be combined using, for example, Maximal Ratio Combining (MRC), which provides significant performance gains for an up-link channel from a mobile handset to a base station.
The second technique for improving communications performance, i.e. polarisation diversity, may be obtained by means of an antenna having two sets of antenna elements, each set providing an angle of maximum signal transmission or reception sensitivity which is orthogonal to that of the other set. Two signals received by such an antenna from a single mobile handset via different multipath routes are at least partly uncorrelated relative to one another, and consequently they may be combined to improve received signal quality.
The third technique for improving communications performance, i.e. variable electrical tilt, arises from the properties of a phased array of antenna elements. Such an antenna forms a main beam in response to in phase excitation of its antenna elements or excitation which varies across the array as a linear function of element position in the array. Changing the gradient of this function changes the inclination or angle of the main beam to the main beam direction which corresponds to in phase excitation. This allows an operator of a cellular mobile radio network to alter the angle of the main beam to the vertical, which in turn adjusts the ground coverage area served by the antenna and enables interference to be reduced between messages associated with different cells: here a cell is a region with which an antenna communicates, and there may be more than one cell per antenna.
Operators of cellular radio network face increasing demand for traffic capacity, but are subject to planning or zoning restrictions which limit deployment of new sites or addition of antennas to existing sites. Consequently it is desirable to increase antenna traffic capacity without adding antennas, e.g. by operators sharing an antenna, while retaining capability for space diversity and adjustment of respective angles of electrical tilt by individual users (operators (Europe), or carriers (USA)) independently of one another.